1.1 Field of the Invention
The invention relates to techniques for reducing and/or correcting for borehole effects encountered in subsurface measurements. More particularly, the invention concerns methods, and devices for their implementation, in which well logging instruments using sources or sensors having a transverse or tilted magnetic dipole are adapted to reduce or correct for undesired electromagnetic effects associated with the deployment of the instruments in a borehole.
1.2 Description of Related Art
Various well logging techniques are known in the field of hydrocarbon exploration and production. These techniques typically employ logging instruments or xe2x80x9csondesxe2x80x9d equipped with sources adapted to emit energy through a borehole traversing the subsurface formation. The emitted energy interacts with the surrounding formation to produce signals that are detected and measured by one or more sensors on the instrument. By processing the detected signal data, a profile of the formation properties is obtained.
Electromagnetic (EM) logging techniques known in the art include xe2x80x9cwirelinexe2x80x9d logging and logging-while-drilling (LWD). Wireline logging entails lowering the instrument into the borehole at the end of an electrical cable to obtain the subsurface measurements as the instrument is moved along the borehole. LWD entails attaching the instrument disposed in a drill collar to a drilling assembly while a borehole is being drilled through earth formations.
Conventional wireline and LWD instruments are implemented with antennas that are operable as sources and/or sensors. In wireline applications, the antennas are typically enclosed by a housing constructed of a tough plastic material composed of a laminated fiberglass material impregnated with epoxy resin. In LWD applications, the antennas are generally mounted on a metallic support to withstand the hostile environment encountered during drilling. Conventional logging instruments are also being constructed of thermoplastic materials. The thermoplastic composite construction of these instruments provides a non-conductive structure for mounting the antennas. U.S. Pat. No. 6,084,052 (assigned to the present assignee) describes implementations of composite-based logging instruments for use in wireline and LWD applications.
In both wireline and LWD applications, the antennas are mounted on the support member and axially spaced from each other in the direction of the borehole. These antennas are generally coils of the cylindrical solenoid type and are comprised of one or more turns of insulated conductor wire that is wound around the support. U.S. Pat. Nos. 4,873,488 and 5,235,285 (both assigned to the present assignee), for example, describe instruments equipped with antennas disposed along a central metallic support. In operation, the transmitter antenna is energized by an alternating current to emit EM energy through the borehole fluid (also referred to herein as mud) and into the formation. The signals detected at the receiver antenna are usually expressed as a complex number (phasor voltage) and reflect interaction with the mud and the formation.
One EM logging technique investigates subsurface formations by obtaining electrical resistivity or conductivity logs by xe2x80x9cfocusedxe2x80x9d measurements. U.S. Pat. No. 3,452,269 (assigned to the present assignee) describes an instrument adapted for taking these focused measurements. The technique described in the ""269 patent uses a survey current emitted by a principal survey current emitting electrode. This survey current is confined to a path substantially perpendicular to the borehole axis by focusing currents emitted from nearby focusing electrodes. U.S. Pat. No. 3,305,771 describes a focusing technique using an instrument equipped with toroidal coils. U.S. Pat. Nos. 3,772,589, 4,087,740, 4,286,217 (all assigned to the present assignee) describe other electrode-type instruments used for subsurface measurements.
U.S. Pat. No. 5,426,368 (assigned to the present assignee) describes a logging technique using an array of current electrodes disposed-on a support. The ""368 patent uses the electrode configuration to investigate the geometrical characteristics of the borehole and the resistivity properties of the formation. U.S. Pat. Nos. 5,235,285 and 5,339,037 (both assigned to the present assignee) describe metallic instruments adapted with a toroidal coil and electrode system for obtaining resistivity measurements while drilling. The measurement techniques described in the ""285 and ""037 patents entail inducing a current that travels in a path including the conductive support body and the formation.
U.S. Pat. Nos. 3,388,325 and 3,329,889 (both assigned to the present assignee) describe instruments equipped with an electrode and coil configuration for obtaining subsurface measurements. U.S. Pat. No. 3,760,260 (assigned to the present assignee) also describes a downhole instrument equipped with electrodes and coils. The ""260 patent uses the electrode configuration to ensure radial current flow into the formation surrounding the borehole. U.S. Pat. No. 4,511,843 (assigned to the present assignee) describes a logging technique whereby currents are emitted from electrodes to zero a potential difference between other electrodes on the instrument. U.S. Pat. No. 4,538,109 (assigned to the present assignee) describes a logging technique aimed at correcting or canceling the effects of spurious EM components on downhole measurement signals.
A coil carrying a current can be represented as a magnetic dipole having a magnetic moment proportional to the current and the area. The direction and strength of the magnetic moment can be represented by a vector perpendicular to the plane of the coil. In conventional induction and propagation logging instruments, the transmitter and receiver antennas are mounted with their axes along the longitudinal axis of the instrument. Thus, these instruments are implemented with antennas having longitudinal magnetic dipoles (LMD). When such an antenna is placed in a borehole and energized to transmit EM energy, currents flow around the antenna in the borehole and in the surrounding formation. There is no net current flow up or down the borehole.
An emerging technique in the field of well logging is the use of instruments incorporating antennas having tilted or transverse coils, i.e., where the coil""s axis is not parallel to the support axis. These instruments are thus implemented with antennas having a transverse or tilted magnetic dipole (TMD). The aim of these TMD configurations is to provide EM measurements with directed sensitivity and sensitivity to the anisotropic resistivity properties of the formation. Logging instruments equipped with TMDs are described in U.S. Pat. Nos. 4,319,191, 5,508,616, 5,757,191, 5,781,436, 6,044,325, 6,147,496, WO 00/50926, and in V. F. Mechetin et al., TEMPxe2x80x94A New Dual Electromagnetic and Laterolog Apparatus-Technological Complex, THIRTEENTH EUROPEAN FORMATION EVALUATION SYMPOSIUM TRANSACTIONS, Budapest Chapter, paper K, 1990.
A particularly troublesome property of the TMD is the extremely large borehole effect that occurs in high contrast situations, i.e., when the mud in the borehole is much more conductive than the formation. When a TMD is placed in the center of a borehole, there is no net current along the borehole axis. When it is eccentered in a direction parallel to the direction of the magnetic moment, the symmetry of the situation insures that there is still no net current along the borehole axis. However, when a TMD is eccentered in a direction perpendicular to the direction of the magnetic moment, axial currents are induced in the borehole. In high contrast situations these currents can flow for a very long distance along the borehole. When these currents pass by TMD receivers, they can cause undesired signals that are many times larger than would appear in a homogeneous formation without a borehole.
U.S. Pat. No. 5,041,975 (assigned to the present assignee) describes a technique for processing signal data from downhole measurements in an effort to correct for borehole effects. U.S. Pat. No. 5,058,077 describes a technique for processing downhole sensor data in an effort to compensate for the effect of eccentric rotation on the sensor while drilling. However, neither of these patents relates to the properties or effects of TMDs in subsurface measurements.
Thus there remains a need for improved methods and apparatus for reducing or correcting for these currents when using well logging instruments implemented with TMDs.
An embodiment of the invention provides an apparatus for use in a borehole traversing a formation. The apparatus comprises an elongated support having a longitudinal axis; at least one antenna disposed on the support such that the magnetic dipole moment of the antenna is tilted or perpendicular with respect to the longitudinal axis of the support. Each antenna is adapted to transmit and/or receive electromagnetic energy. First and second electrodes are disposed on the support, with the second electrode being disposed such that at least one antenna is located between the first and second electrode. The first electrode is coupled to the second electrode to provide a path for a current between the electrodes.
Another embodiment of the invention provides an apparatus for use in a borehole traversing a formation. The apparatus comprises an elongated non-conductive support having a longitudinal axis and at least one conductive segment disposed thereon. At least one antenna is disposed on the support such that the magnetic dipole moment of the antenna is tilted or perpendicular with respect to the longitudinal axis of the support. The at least one antenna is disposed along a conductive segment on the support; and each antenna is adapted to transmit and/or receive electromagnetic energy.
Another embodiment of the invention provides an apparatus for use in a borehole traversing a formation. The apparatus comprises an elongated support having a longitudinal axis and at least one antenna disposed on the support such that the magnetic dipole moment of the antenna is tilted or perpendicular with respect to the longitudinal axis of the support. Each at least one antenna is adapted to transmit and/or receive electromagnetic energy. A first pair of electrodes is disposed on the support and adapted for joint electromagnetic interaction. The first pair of electrodes is disposed such that the at least one antenna is located between the electrodes. A second pair of electrodes is disposed on the support and adapted for joint electromagnetic interaction. The second pair of electrodes is disposed such that the first electrode pair is located between the second electrode pair.
Another embodiment of the invention provides a method for altering the flow of an axial electric current along a subsurface borehole in the vicinity of an antenna disposed within the borehole, the antenna being disposed such that the magnetic dipole moment of the antenna is tilted or perpendicular with respect to the borehole axis and being adapted to transmit and/or receive electromagnetic energy. The method comprises providing first conductive means within the borehole; providing second conductive means within the borehole, the second conductive means positioned such that the antenna is located between the first and second conductive means, and coupling the first and second conductive means to provide a path through the antenna for the axial current to flow between the conductive means.
Another embodiment of the invention provides a method for altering the flow of an axial electric current along a subsurface borehole in the vicinity of an antenna disposed within the borehole, the antenna being disposed on a non-conductive support having a longitudinal axis and adapted for disposal within the borehole, the antenna being adapted to transmit and/or receive electromagnetic energy. The method comprises mounting a conductive segment on the support such that the segment is exposed to the borehole when the support is disposed within the borehole; disposing the antenna along the conductive segment to provide a path through the antenna for the axial current flow when the support is disposed within the borehole; and disposing the antenna along the conductive segment such that the magnetic dipole moment of the antenna is tilted or perpendicular with respect to the longitudinal axis of the support.
Another embodiment of the invention provides a method for altering the flow of an axial electric current along a subsurface borehole in the vicinity of an antenna disposed within the borehole, the antenna being disposed such that the magnetic dipole moment of the antenna is tilted or perpendicular with respect to the borehole axis and being adapted to transmit and/or receive electromagnetic energy. The method comprises disposing a first pair of electrodes within the borehole such that the antenna is located between the electrodes, the first electrode pair being adapted for joint electromagnetic interaction; disposing a second pair of electrodes within the borehole such that the first electrode pair is located between the second electrode pair, the second electrode pair being adapted for joint electromagnetic interaction; measuring an electromagnetic property associated with the axial electric current at the first or second electrode pair; and emitting a current within the borehole in response to the measured electromagnetic property, the current being emitted between: i) the first electrode pair if the second electrode pair was used in the measurement of the electromagnetic property; or ii) the second electrode pair if the first electrode pair was used in the measurement of the electromagnetic property.
Another embodiment of the invention provides a method for correcting for the effect of an axial electric current flow along a subsurface borehole in the vicinity of a receiver antenna disposed within the borehole, the receiver antenna being disposed such that the magnetic dipole moment of the antenna is tilted or perpendicular with respect to the borehole axis, the axial current being associated with electromagnetic energy transmitted from a transmitter antenna disposed within the borehole. The method comprises disposing a first pair of electrodes within the borehole such that the antenna is located between the electrodes, the first electrode pair being adapted for joint electromagnetic interaction; disposing a second pair of electrodes within the borehole such that the first electrode pair is located between the second electrode pair, the second electrode pair being adapted for joint electromagnetic interaction; measuring a voltage signal associated with the transmitted electromagnetic energy at the receiver antenna; measuring a voltage difference between the first or second electrode pair, the voltage difference being associated with the transmitted electromagnetic energy; shutting off transmission of the electromagnetic energy from the transmitter antenna; emitting a current within the borehole, the current being emitted between: i) the first electrode pair if the second electrode pair was used in the voltage difference measurement; or ii) the second electrode pair if the first electrode pair was used in the voltage difference measurement; measuring a voltage signal associated with the emitted current at the receiver antenna; measuring a voltage difference between the electrode pair used in the previous voltage difference measurement, the voltage difference being associated with the emitted current; and calculating a voltage value based on the measured voltage signals and voltage differences.
Another embodiment of the invention provides an apparatus for induction logging within a borehole traversing a formation. The apparatus comprises an elongated conductive metal body having a longitudinal axis and at least one antenna disposed on the body such that the magnetic dipole moment of the antenna is tilted or perpendicular with respect to the longitudinal axis of the body. Each antenna is adapted to transmit and/or receive electromagnetic energy for electromagnetic exploration of the formation.
Another embodiment of the invention provides a method for altering the flow of an axial electric current along a subsurface borehole in the vicinity of an antenna disposed within the borehole, the antenna being disposed on a conductive metal body having a longitudinal axis, the antenna being adapted to transmit and/or receive electromagnetic energy at a frequency range of 1 kHz to 5 MHz. The method comprises disposing the antenna along the metal body such that the magnetic dipole moment of the antenna is tilted or perpendicular with respect to the longitudinal axis of the body; and disposing the metal body within the borehole such that the body is exposed to the borehole to provide a path for the axial current flow.